Unidirectional wave transducer

ABSTRACT

The invention concerns unidirectional ground-wave transducers using an alternating series of emitter cells (E) and reflector cells (R). 
     The invention consists in using, in the emitter cells (E), fingers having a width of λ/3 spaced apart by a distance of 2 λ/3; and, in the reflector cells (R), fingers having a width of λ/4 spaced apart by a distance of λ/2. 
     Using identical engraving precision, the invention makes it possible to increase by 50% the frequency at which the unidirectional transducer thus produced can be used.

The present invention concerns unidirectional ground wave transducersallowing propagation along the surface of a substrate of a ground wave,in such a way that the main part of the signal is emitted through oneend of this transducer, and that any signal emitted from the other endof the transducer is of a very low level in comparison with the mainsignal.

The current state of the art encompasses various methods for producingtransducers of this kind, for example by using floating electrodes thatare engraved with an accuracy of more than λ/8, λ being the wavelengthat the midband frequency of the frequency band within which thetransducer is used, or metal or dielectric layers superposed on theelectrode metal plating. These methods are costly, and difficult toimplement.

To solve these problems, the Applicant, in French Patent Application No.89 13747, filed Oct. 20, 1989 and published Apr. 26, 1991 under number2,653,632, proposed making a alternating series of emitter and reflectorcells, in which the emitter cells comprise fingers having a widthsubstantially equal to λ/6 and spaced apart by a distance equal to λ/3;the reflector cells have fingers approximately λ/4 in width and arespaced apart by λ/2.

While the advance represented by this structure is unquestioned, theprecision entailed by engraving so as to obtain a spacing of λ/6 musttherefore be great, and, in consequence, not easily achieved.

To solve this difficulty, the invention relates to a transduceraccording to claim 1.

Other features and advantages of the invention will clearly emerge fromthe following description provided as non-limiting examples withreference to the attached drawings, in which:

FIG. 1 is a top view of the conventional skeleton structure of aunidirectional transducer;

FIG. 2 is a top view of an emitter cell according to the invention;

FIG. 3 is a top view of a reflector cell used in the invention;

FIG. 4 is a cross-sectional view of a portion of the transduceraccording to the invention;

FIG. 5 is a view similar to that in FIG. 4, but which clarifies a numberof explanations;

FIG. 6 is a cross-sectional view of a portion of a transducer accordingto one variant of the invention;

FIG. 7 is a partial top view of another variant of a transduceraccording to the invention.

FIG. 1 illustrates an example showing the overall structure of theelectrodes in an operating acoustic reflection transducer, similar tothose placed on the surface of a piezoelectric substrate (not shown inthe figure).

These electrodes are illustrated by rectangles containing excitationmeans (not shown), which normally exist as interdigitated combs. One ofthese combs is grounded and the other is connected to an electricalconnection S. When an electric signal is applied at S, an acoustic waveis generated at each of the left (G) and right (D) acoustic ports formedby the ends of the transducer. This unit is, moreover, reversible, andthe acoustic waves propagated on the substrate surface may enter throughthese ports and energize the transducer so that it will supply anelectric signal to the connection S.

The electrodes are combined as emitter cells E₁ to E₃ and as reflectorcells R₁ to R₄ aligned in alternating fashion along the axis ofpropagation of the acoustic waves. Only the emitter cells connected tothe electrical connection S energize the acoustic waves on the substratesurface, while the reflector cells modify the acoustic propagationcharacteristics along this axis.

As in the aforementioned French patent application, the invention isintended to produce, based on the structure of the emitter and reflectorcells and on their respective arrangement, a substantially nilelectro-acoustic transfer between the electric input port S and one ofthe two left or right acoustic ports G or D. This zero transfer producesa single-phase unidirectional transducer.

FIG. 2 illustrates two views of an emitter cell E, a top view in theupper part of the figure and a cross-sectional view corresponding to thetop view, in the lower part thereof. The two electrodes forminterdigitated combs. The grounded electrode 41 incorporates twofingers, and the electrode 42 connected to the terminal S comprises onlyone. These fingers are spaced apart by P_(E) =2λ/3, λ being thewavelength at the operating midband frequency of the device. The nominalwidth of the fingers is λ/3, but this value is not imperative. Thesuccession of fingers is such that the single finger of the electrode 42is followed by the two fingers of the electrode 41, going from left toright. To properly delimit the emitter cell with reference to itsposition in FIG. 1, the conventional boundaries, both left 43 and right44, will be specified. The left boundary 43 is located λ/2 to the leftof the center of the single finger of the electrode 42, and the rightboundary 44, 3λ/2 to the right of this center. The total length of thecell between its left and right boundaries is, therefore, 2λ.

Because of the type of connection to the electric signal source allowingthe cell to be energized, the electrode 41, which is grounded, is a coldelectrode and the electrode 42, a hot electrode, as understoodradioelectrically by the use of the symbols - and +. It may also be saidthat the + electrodes are "active," and the - electrodes "inactive."Under these conditions, the emission center of the acoustic waves islocated in the center of the single finger belonging to the electrode42. By virtue of the specification of the left and right boundaries, theacoustic waves emitted by the single finger of the electrode 42 exhibita phase difference of 360° at these two boundaries, and are thus "inphase."

The cell illustrated in FIG. 2 is the simplest cell that can be producedaccording to the invention. It thus represents merely one very specific,non-limiting example of an emitter cell. It is possible, especially toincrease the emission efficacy of the acoustic waves, to enlarge thecell by duplicating it exactly using a 2λ translational shift. The cellsthus produced by duplicating the configuration N times will comprise Nfingers on the electrode 42 and 2N fingers on the electrode 41. Thesefingers will occur in succession according to the sequence specified inFIG. 2. All of these cells will possess the same basic characteristic asdoes the elementary cell in FIG. 2: that is, the acoustic wave will bein phase at the two boundaries, left and right, of the cell. Moreover,within a multi-component cell of this type, a number of fingersbelonging to the electrode 42 could be eliminated, for example byconnecting them to the electrode 41, in order to weight conventionallythe emitted signal so as to perform, for example, specific filteringfunctions.

FIG. 3 illustrates, using the same conventions as in FIG. 2, a reflectorcell comprising two finger-shaped electrodes 51 and 55. These electrodesare spaced apart by P_(R) =λ/2. Their nominal width is λ/4, but thiswidth is not imperative. They may be either isolated or grouped togetherand grounded, that is, in this last case, in a short-circuit. As in thecase of the transmitter cell, a left boundary 53 located at 3λ/8 to theleft of the center of the electrode 51 and a right boundary 54 locatedλ/8 to the right of the center of the electrode 55 are specified. Eachof these electrodes acts as an elementary reflector, and it is knownthat the reflection coefficient of each of these reflectors is purelyimaginary, if the axis of symmetry of this reflector is taken as thephase reference for incident and reflected waves. In other words,reflection exhibits a phase shift of + or -90° . The determination of +or - depends, conventionally, on the physical properties of thematerials and of the substrate, in conjunction with the fact that thefingers are isolated or combined in a short-circuit. In the mostfrequently-occurring case, the sign + indicates that the fingers areshort-circuited, and the signal -, that the fingers are isolated. In theremainder of the description, the sign + will be considered, but theinvention is also applicable when the sign is -.

Under these conditions and taking as reference point the left boundary53, the phase difference at this left boundary between the incident waveand the wave reflected on the first reflector 51 is 360°, or 0°, since a90° phase shift occurs as a result of reflection and a 270° shiftresults from the back-and-forth movement over a distance of 3λ/8 betweenthe left boundary and the center of the reflector. Because the distancebetween the reflectors is λ/2, the same calculation can be made withrespect to the reflector electrode 55. The wave reflected on the latterwill be in phase with the wave reflected on the electrode 51, and,therefore, in phase with the incident wave at the left boundary 53.Because the reflection coefficient of a single electrode is low and,accordingly, since a large portion of the incident wave coming from theleft continues to be propagated toward the right after travelling acrossthe site of an electrode, the reflection coefficient of two electrodes,as in the single reflector cell illustrated in the figure, will be twotimes that of an isolated electrode. Similarly, in the case of a moreextended reflector cell comprising, for example, m reflector electrodes,the total reflection coefficient of this cell will be approximately mtimes the reflection coefficient of a single electrode.

When the acoustic waves come from the right, they are also reflected onthe reflectors and, when the right boundary 54 is taken as the phasesource, the length of the back-and-forth movement on the first electrode55 equals λ/4, since the center of reflection, i.e., the middle of thiselectrode, is separated by a distance of λ/8 from the right boundary 54.Taking into account the 90° phase shift resulting from reflection, thephase difference between the incident wave and the reflected wave at theright boundary 54 is 180° . The same situation is true to within 360° asregards reflection on the electrode 51 in FIG. 3 and on any otheradditional electrode, when a more extended reflector cell is chosen. Thesum of the reflected waves on each of these electrodes produces theentire reflected wave in the same way as that described above.

In short, taking the left boundary 53 as the phase reference point, thereflection coefficient of an incident wave coming from the left has aphase of 0°. Taking the right boundary 54 as the phase reference point,the reflection coefficient for an incident wave coming from the righthas a phase of 180°.

FIG. 4 is a cross-sectional view of a cell E₁ of the type describedabove, bounded by two reflector cells R₁ and R₂ of the type describedabove. This figure also illustrates the cross-section of a cell E₂ andthe beginning of two other reflector cells on the right and left, inorder to show clearly that E₁, R₁, and R₂ belong to a unit in relationto which the description of operation is identical to the followingdescription for E₁, R₁, and R₂.

These cells are arranged in such a way that the left boundary of E₁corresponds to the right boundary of R₁, and the right boundary of E₁,to the left boundary of R₂. These conventional boundaries were delimitedprecisely for this reason, i.e., to make it possible to more easilydescribe the respective positions of the emitter and the reflectorcells. Accordingly, there is a shared boundary 43-54 between R₁ and E₁and a shared boundary 44-53 between E₁ and R₂.

The acoustic waves 61,G and 61,D emitted by E₁ from the center of theelectrode 42 are emitted symmetrically from both sides of this electrodetoward the boundaries of E₁.

With respect to the cell R₂, the wave 61,D is incident from the left. Itis, therefore, partially reflected with zero phase shift at the boundary44, 53 in the form of a wave 62.

After propagation over a length of 2λ, this wave 62 reaches the boundary43, 54. Since, as indicated above, the phase of the waves 61,G at theboundary 43, 45 and 61,D at the boundary 43, 54 are the same, the waves62 and 61,G have the same phase.

on the other hand, the incident wave 61,G coming from the right at thecommon boundary 43, 54 is reflected there in the form of a wave 63incorporating a phase shift of 180°, as was indicated above.Accordingly, this wave is in opposition of phase at the other sharedboundary 44, 53 in relation to the wave 61,D. These waves 63 and 61,Dthus produce destructive interference, thereby attenuating the residualwave being propagated toward the right side of the figure. On the whole,the elementary unit formed by an emitter cell E₁ bounded by tworeflector cells R₁ and R₂ acts as a transducer, which emits toward theleft an acoustic wave whose amplitude is greater than that of theacoustic wave emitted toward the right. Repetition of this elementaryunit a certain number of times produces a transducer which issubstantially unidirectional in its entirety. Indeed, because thereflector cells R₁ to R_(N) are separated from each other by a wholenumber of half-wavelengths, they generate reflected waves all of whichare in phase, thereby intensifying the property described above. Theemitter and reflector cells do not, moreover, all have to be identical;it suffices that the phase conditions at the boundaries be the same asthose specified in the basic cells described above.

It will also be noted that, if the reflection coefficient inherent ineach elementary reflector belonging to the reflector cells is -90°, theoperation of the unit is reversed; and that a left-to-right instead of aright-to-left unidirectional transducer is obtained.

It is known, moreover, that an interdigitated transducer emitsreciprocally-harmonic frequencies corresponding to occurrences in whichthe waves emitted by the active fingers are put back in phase. Thus, fora transducer comprising several cells similar to those illustrated inFIG. 2, the emitted frequencies correspond to the λ wavelengths given bythe formula:

    kλ=3P.sub.E.

This is because the distance between two + electrodes is 3P_(E). Thus,it is found that at least two frequencies F₁ and F₂ are emitted, thewavelengths of which are given by:

    λ.sub.1 =3P.sub.E

    λ2=3P.sub.E /2

As described above, in accordance with the invention, a spacing of P_(E)=2λ₀ /3 is chosen for the wavelength λ0. The wavelengths λ₀ and λ₂ arethus identical. Under these conditions, therefore, the emitter cellswill not only be in phase at the frequency F₀ corresponding to λ₀, butalso at the frequency F₀ /2 and, in addition, at all frequencies whichare multiples of F₀ /2. It may nevertheless be shown that thefrequencies which are multiples of 3F₀ /2 are not coupled. The frequencyF₀ /2 in fact corresponds to a subharmonic, which presents practicaldifficulties. Indeed, if the piezoelectric material composing thesubstrate holding the combs is quartz, this subharmonic F₀ /2 causesgeneration of a bulk wave at frequencies approaching 1.6.F₀ /2 and1.8.F₀ /2 by virtue of a known physical phenomenon. Because thesefrequencies are very close to F₀, they produce interference lines whichcause deterioration of filter rejection.

To ensure that frequencies at the wavelength λ1, which may thus excitebulk waves, are not emitted, there must be no return to the in-phaseconfiguration at that frequency from one emitter cell to the next. FIG.5 illustrates two emitter cells E₁ and E₂ of the type previouslydescribed, which are separated by two reflector cells R₁ and R₂, also ofthe kind described above. The usable frequency to be emitted is λ₂, andthe interference frequency to be avoided is λ₁. To eliminate thisinterference frequency, the centers of phase of the emitter fingers 42of the two cells E₁ and E₂ must be separated by a distance equal to anodd number of repetitions of λ₁ /2. As shown in the figure, thiscondition is fulfilled precisely in the case of emitter and reflectorcells corresponding to FIGS. 2 and 3, which are used in the operatingreflector shown in FIG. 4, since, in this case, this distance betweenthe two centers of phase is equal to 3λ₂ =1.5λ₁. This condition can beapplied generally by expressing it as a requirement, according to whichthe interval between successive emitter cells must be equal to2(2m+1)/λ₂ ; that is, to an odd multiple of two wavelengths at thefrequency employed. In the case demonstrated, m=0, and the odd multipleis 1.

In addition, as shown in FIG. 6, it is also possible to use somereflector cells having an uneven number of fingers, e.g., the cell R₁having a single finger. Accordingly, the waves emitted, on the one hand,by the emitter cell E₁ and, on the other, by the emitter cells E₂ and E₃will be in opposition of phase at the left boundary 43 of the emittercell E₁ at the frequency corresponding to the usable wavelength λ2. Thismakes it possible to obtain both positive and negative components bychoosing this odd number of reflectors for a reflector cell, and,therefore, to obtain positive and negative weights when use is made of amethod for weighting the emitter cells by source suppression. Thismethod is known in English terminology as "withdrawal." Finally, use maybe made a structure allowing a differential power feed between a +source and a - source, in relation to ground, as shown in FIG. 7. Thisstructure comprises two emitter cells E₁ and E₃ connected to the +source, which enclose an emitter cell E₂ connected to the - source,these three cells being separated by the potentially-grounded reflectorcells R₁ and R₂, these cells being, of course, placed at groundpotential by symmetry.

This configuration allows the use of an intricately-patterned structurein which the grounded electrodes are interposed between those connectedto the + source and those connected to the - source. In this way,enhanced protection is gained against electrostatic and pyroelectricinterference.

All of the structures described above have the advantage that they canbe produced using a single level of metallization and a singlephotoengraving operation. In comparison with conventional structuresproduced to date, in which photoengraving precision had to reach λ/6,the level of precision here is limited to λ/4 and λ/3, thereby making itpossible, at the same level of photoengraving precision, to obtain adevice that can operate at frequencies 50% higher than those normallyused.

I claim:
 1. Unidirectional ground wave transducer of the type comprisinga line of emitter cells (E) and reflector cells (R) alternating insuccession and arranged in such a way that the acoustic waves emitted bythe emitter cells and reflected by the reflector cells are addedtogether constructively in a direction of propagation and destructivelyin the other direction of propagation, wherein the emitter cells consistof two electrodes (41, 42) existing as interdigitated combsincorporating a spacing (P_(E)) equal to 2λ/3, λ being the operatingmidband wavelength of the transducer.
 2. Transducer according to claim1, wherein the electrode fingers in the emitter cells (41, 42) have awidth approaching λ/3.
 3. Transducer according to claim 2, wherein theelectrodes (41, 42) of each active cell consist of at least one groupingof three fingers, including an active finger (42) followed by twoinactive fingers (41).
 4. Transducer according to claim 3, wherein theemitter cells are composed of a succession of units of three fingersconsisting of one active finger followed by two inactive fingers. 5.Transducer according to any of claims 1 to 4, wherein the reflectorcells (R) comprise fingers (51, 55) spaced apart by an interval of λ/2and have a width approaching λ/4, the adjacent emitter cells beingpositioned in such a way that the center of the closest active finger islocated at a distance of ##EQU1## from the centers of the reflectors onone side, and ##EQU2## on the other side.
 6. Transducer according toclaim 5, wherein most of the emitter cells are separated by an oddnumber of wavelengths.
 7. Transducer according to claim 6, wherein atleast one of the reflector cells (R₁) comprises an odd number of fingersmaking it possible to produce sources having opposite signs. 8.Transducer according to any of claims 1 to 7, wherein, from one emittercell to the next, the active electrodes are connected in succession to apositive source (S+), then to a negative source (S-), these activeelectrodes being separated by an odd number of half-wavelengths, and theinactive electrodes in the emitter cells, as well as the fingersbelonging to the reflector cells (R₁, R₂) are connected and potentiallygrounded, by using an intricately-patterned configuration making itpossible to isolate electrostatically and pyroelectrically theelectrodes connected to the positive source from the electrodesconnected to the negative source.